Chox
In answer to your original question,
JAR–FCL 1.215 Class ratings (A) states:-
[(1) Class ratings for aeroplanes will be issued according with the associated administrative procedures accepted by the JAA. In order to change to another type or variant of the aeroplane within one class rating, differences or familiarisation training is required.];
LASORS Appendix E to Section F gives guidance on differences training.
No minimum times are specified.
Mike